Electric power steering (EPS) systems for vehicles such as automobiles and trucks typically include a steering wheel, an electric motor, a controller, one or more sensors, a steering shaft, and a steering gear assembly. The steering gear assembly can be a rack and pinion gear assembly, a recirculating ball steering gear assembly or any other suitable steering gear assembly. The electric motor is typically coupled to the steering shaft through a worm that is connected to the motor and a worm gear that is connected to the steering shaft.
The sensors typically include a torque sensor that provides a feedback signal to the controller. The feedback signal represents driver effort that is required to turn the steering wheel. As the driver effort increases, the electric motor rotates the worm that engages and rotates the worm gear. The worm gear is connected to the steering shaft and reduces driver effort that is required to turn the steering wheel. Other sensed parameters typically include a rotational sensor that senses steering shaft rotational position and that provides a feedback signal to the controller. Vehicle velocity is also typically input to the controller so that the assist provided by the EPS system varies as a function of vehicle speed.
EPS systems offer improvements over conventional hydraulic assist systems by reducing overall vehicle weight and improving fuel economy. In addition, EPS systems allow for precise electric control of the steering system. In addition to variable effort assist, the EPS systems can also provide steering wheel return characteristics that may be tuned to a desired feel and/or responsiveness. The amount of tactile feedback to the driver through the steering wheel may also be electrically controlled. Specifically, the steering torque provides information to the driver regarding road conditions and vehicle maneuverability. The amount of restoring torque is a function of the chassis design and the transmissibility of rack loads back to the steering wheel. The EPS systems provide active control of the transmissibility characteristics and therefore the amount of tactile feedback to the driver.
The electric motors that are used in EPS systems should have low levels of cogging torque, low torque ripple, and high torque density. Conventional permanent magnet motors that have been employed in EPS systems generally have a 1.5 slot/pole ratio (such as 6/4, 12/8, and 18/12), or a 3.0 slot/pole ratio (such as 36/12). The motors with the 1.5 slot/pole ratio have high magnitude, low-frequency cogging torque and large magnitude torque ripple. The motors with the 3.0 slot/pole ratio have high magnitude, low-frequency cogging torque, large magnitude torque ripple, and lowpower density. Skewing of the rotor or stator is usually required to reduce the cogging torque. The addition of the rotor or stator skew reduces the power density of the motor and increases the cost of both materials and manufacturing. For example, skewing the rotor generally involves the use of permanent magnets that have a complex shape. These permanent magnets are difficult to manufacture and to handle during the assembly process, which increases the manufacturing costs of the motor. The cost penalties that result from rotor or stator skew become greater as the amount of skew is increased. In general, the amount of skew increases with decreasing cogging torque frequency.
Therefore, an EPS system that includes a permanent magnet motor with relatively low cogging torque, low torque ripple and high power density is desirable. The permanent magnet motor should be assembled and manufactured at a relatively low cost as compared to the conventional permanent magnet motors described above.